The technical field of this invention is the control of rear wheel steer angle in vehicles.
All motor vehicles are provided with steering gear responsive to driver input and/or other inputs to turn selected wheels of the vehicle for the purpose of changing vehicle direction. Historically, the selected wheels have generally been the front wheels, particularly for passenger vehicles capable of a wide range of forward speeds. Recently, interest is growing in the use of rear wheel steering to complement front wheel steering and extend the range of control of vehicle dynamics at widely different vehicle speeds. For example, desired vehicle steering characteristics are very different in low speed parking situations than in high speed highway cruising or performance driving. Since most drivers are accustomed to the behavior of vehicles having front wheel steer, vehicles with front and rear wheel steer generally provide direct driver control of the front wheel steering angle while the control automatically directs the rear wheels in a predetermined manner to complement the front wheel direction. This invention is primarily concerned with a method and apparatus for providing such rear steer control.
The method and apparatus of this invention determine, when desired and actual lateral accelerations of the vehicle have the same direction, an instantaneous dynamic yaw center on a vehicle longitudinal axis in response to the measured vehicle speed and a cornering radius. They further determine a rear wheel steering angle corresponding to a wheel direction perpendicular to a straight line connecting the center of the rear wheel to an instantaneous Ackerman center defined at a distance of the cornering radius from the dynamic yaw center in a direction perpendicular to the vehicle central longitudinal axis and direct a rear wheel of the vehicle to be perpendicular to a line connecting the instantaneous Ackerman center to the center of the wheel. The dynamic yaw center, and thus the instantaneous Ackerman center, is preferably shifted rearward with increasing vehicle speed in order to shift vehicle steering in a direction from oversteer at very low speeds for greater ease of steering in parking maneuvers to understeer for vehicle stability in high speed driving conditions. The method and apparatus preferably include a reverse lock routine to control the rear wheels when the desired and actual lateral accelerations of the vehicle have opposing directions, to control the vehicle in a four wheel drift; and the reverse lock routine may also control suspension dampers to further optimize steering characteristics.